1. Field of the Invention
This invention relates to a reciprocating piston engine and, more particularly, to a piston engine with two piston rings, namely top and second compression rings.
2. Description of the Prior Art
A typical piston of an internal combustion engine is provided with three piston rings comprising two compression rings and one oil-control ring for preventing blowby (leakage of high pressure combustion chamber gases into a crankcase) and for preventing oil from the crankcase from passing into the combustion chamber to be consumed.
On the other hand, pistons with only two compression rings have been recently developed to decrease frictional resistance and weight (by reducing piston size) as disclosed in, for example, U.S. Pat. No. 4,362,136. However, since simply reducing the number of piston rings involves the occurence of blowby and aggravation of oil consumption, special designs in the shapes of the piston rings are proposed as disclosed, for example, in Japanese Utility Model Laid-Open (KOKAI) Nos. 59-126158 59-65958.
A top ring, one of the compression rings positioned toward a piston head, disclosed in Japanese Utility Model Laid-Open No. 59-126158 has the following two facing ends: one end has a protrusion extending at a corner in the outer periphery of the ring and closer to the combustion chamber, while the other end has a recess facing the protrusion and receiving the same when the ring is placed in a ring groove. However, such a piston ring has the disadvantages that manufacturing, costs are increased because of the complicated shape of the facing ends and that the protrusion could easily be damaged because of its small sectional area and therefore low stiffness and because of a strength decrease of the top ring resulting from direct exposure to high-temperature combustion gases.
Also, a piston disclosed in Japanese Utility Model Laid-Open No. 59-65958 has top and second piston rings of a usual rectangular section. A through hole is formed in the piston to communicate a space defined between the top and second rings to the backside of the piston, i.e. to the crankcase side, when the top ring abuts against an upper surface of its ring groove.
However, in the above discussed prior art arrangements no consideration is given to the behaviors of the piston rings relative to the reciprocating movement of the piston. Taking into consideration the behaviors of the piston rings, which change according to operating conditions of the engine, as discussed hereinbelow, these pistons are found still unsatisfactory to reduce oil consumption.
Experiments by the present inventors have revealed that the behaviors of the piston rings relative to the up-and-down movement of the piston are quite different from what is conventionally believed, and the movement of the rings change according to number of revolutions and loaded conditions of the engine.
The behaviors of the piston rings have heretofore been considered to be controlled generally by the friction force along an inner wall of the cylinder and the inertia force of the piston rings so as to follow belatedly the up-and-down movement of the piston. In other words, it has been believed that the piston rings abut against lower surfaces (the surfaces farther from the piston head) of ring grooves when the piston moves upward, and against upper surfaces thereof (the surfaces closer to the piston head) when the piston moves downward. However, since negative pressure acts on the piston rings during the intake stroke, and since combustion pressure acts thereon during the power stroke, the behaviors of the piston rings cannot simply be related to the interaction between the friction and inertia forces. Furthermore, since these pressures vary according to the working conditions of the engine, the behaviors of the piston rings become very complicated.
It is a well known fact that oil consumption is maximum when the engine runs at high speed with high load. Therefore, it is particularly effective if oil consumption can be reduced during such an operating condition.
The present inventors have found that positively leading the high combustion pressure during the power stroke into a space defined by the top and second rings, the inner surface of the cylinder and the outer periphery of the piston (hereinafter referred to as "second land clearance") is very effective in reducing oil consumption. However, in the piston disclosed in the above Japanese Utility Model Laid-Open No. 59-65958, even if the combustion pressure could be introduced into the second land clearance by, for example, providing a passage for communicating the combustion chamber to the second land clearance, the pressure would be released to the crankcase via the through hole, resulting in a failure to increase the pressure in the second land clearance.
The experiments have further revealed that the more the pressure in the second land clearance increases, the lower will be the amount of oil consumption when the engine runs at high speed with high load. On the contrary, with high pressure in the second land clearance, the oil consumption increases when the engine runs at low speed with small load, that is, when a negative pressure is large during deceleration and idling.